Regulation of the type III inositol 1,4,5-trisphosphate receptor

The inositol 1,4,5-trisphosphate receptor (InsP3R) is an important intracellular calcium (Ca2+) release channel in a variety of tissues. InsP3-mediated Ca2+ release regulates many cellular processes such as muscle contraction, transcription, and long term depression. Three isoforms have been identified, but only the functional properties of the types I and II InsP3R have been determined. This dissertation examines the regulation of the type III InsP3R and the physiological role of the type III InsP3R in an intact cell. A rat insulinoma cell line, RIN-m5F, was used as a source for the type III InsP3R since these cells preferentially express this isoform. Chapter 1 provides a brief description of normal pancreatic β cells and RIN-m5F cells as well as the clinical consequences of altered insulin secretion. In chapter 2, regulation of the type III InsP3R by Ca2+ is explored. An important functional difference is presented: the type III InsP 3R is not inhibited by high cytoplasmic Ca2+ like the type I InsP3R. Regulation of the type III InsP3R by InsP3 and ATP is described in chapter 3. The type III InsP 3R requires higher InsP3 concentrations to reach maximal activity relative to the type I InsP3R. This result is consistent with the relative order of InsP3 affinity for the three InsP 3R isoforms. ATP, an important component of excitation-secretion coupling in the β cell, regulates the type III InsP3R in a manner which is very similar to the type I InsP3R. In chapter 4, the nature of Ca2+ signaling in RIN-m5F cells is explored. The type III InsP3R lacks Ca2+-dependent inhibition, and this property makes the type III InsP3R ideal for signal initiation. Activation of the type III InsP3R results in a single transient, but global, increase in the cytosolic Ca2+ concentration.